With a mobile/satellite communications network, such as the Connexion By Boeing™ system, disclosed in U.S. patent application Ser. No. 09/989,742, the disclosure which is hereby incorporated by reference, high speed data from a wide area network (such as the Internet) and various entertainment content is supplied to users and operators of commercial and business aircraft. However, such mobile/satellite networks are not limited to only aircraft, and may just as readily include land vehicles, such as trucks and trains, or mobile platforms such as ships and yachts.
The Connexion By Boeing™ system mentioned above is comprised of generally four segments: a space segment which consists of a leased, fixed satellite service (FSS) transponders, an aircraft earth station segment (i.e., a mobile platform segment) which consists of RF transceiver terminals installed on each aircraft that operates within a given coverage region, a ground-based earth station segment which consists of one or more Fixed Satellite Services (FSS) earth stations, and a network operation center (NOC) which controls the aggregate emissions from the RF transceivers on each of the mobile platforms in order to prevent interference to other satellite-based transponders orbiting in the vicinity of the FSS transponder which is linking the FSS earth station with the mobile platforms. This communications system is shown in FIG. 1. The ground station is in communication with the NOC, preferably via redundant high speed data connections. Multiple ground stations may be included and operate on “stand-by” for redundancy purposes.
With reference to FIG. 2, the communication link with the mobile platform consists of two parts: one or more forward RF links and a return RF link. Each forward RF link carries data from the ground station via the satellite based FSS transponder to the transceivers located on each of the mobile platforms at a high data rate (up to five Mbps or greater). A forward link consists of a single carrier per forward link transponder. Multiple mobile platforms receive the forward link signal, demodulate the package stream of information that it carries, and sort out the packets that are addressed to that particular mobile platform. Each mobile platform may receive signals from multiple forward link transponders. The return link signal from each mobile platform carries data from that particular mobile platform via the satellite based FSS transponder to the ground station. In one preferred form, separate transponders are used for the forward link and return link signals. Each mobile platform transmits data via its return link signal at varying rates, but typically between 16K bps and 1024K bps. Typically, a satellite based FSS transponder is shared by multiple mobile terminals using one or more well known frequency spreading techniques.
In general, it is a goal of a mobile/satellite communication system as described above to maximize the data rate of forward link transmissions to the mobile platforms while maintaining a sufficient “link margin” at all points in a desired coverage area to close the communications link. Link margin is the difference between the measured Eb/No (energy per bit divided by noise power spectral density) received at the ground station and the threshold value of Eb/No at which the communications link is dropped. Transmitting forward link signals at the highest data rates possible while maintaining a sufficient link margin to close the communications link with the mobile platform makes the best (i.e., most cost efficient) use of expensive satellite-based FSS transponders. However, it will be appreciated that increasing the data rate decreases the Eb/No, and therefore the link margin, at any point within a given coverage region. Thus, the link margin of the forward link signals to the mobile platforms operating within a given coverage region will fall as the data transmission rate is increased. This is because the FSS satellite has a fixed transmit power, so increasing the data rate decreases the energy per bit (Eb) and, therefore, Eb/No and link margin. Eventually, an unacceptably high level of bit errors will occur for the signal received by the mobile platforms when the Eb/No falls below a threshold value. At this point, the link will be considered as having dropped (i.e. failed).
When selecting an appropriate forward link data transmission rate, the system designer must contend with a number of factors that cause Eb/No, and therefore link margin, to vary with the location of the mobile platform. The Eb component of Eb/No received at the mobile platform transceiver is influenced by the satellite equivalent isotropic radiated power (EIRP), which may vary by several dB over the coverage area of the satellite. The Eb component is also influenced by the gain of the antenna used on the mobile platform. When a phased array antenna is used on the mobile platform, then it will be appreciated that the gain of such an antenna will decrease with increasing scan angle. A mobile platform that is operating farther from a given satellite/based ESS transponder will have a higher scan angle from zenith to the satellite and therefore a lower antenna gain and, thus, a lower received Eb and link margin. The noise power spectral density (No) component of Eb/No is influenced by the interference noise the mobile platform receives from satellite transponders associated with satellites operating in proximity to the FSS transponder(s) that the mobile platform is attempting to communicate with. This interference varies with location because adjacent satellites also have EIRP patterns that vary with location. Increased interference increases No, which in turn decreases Eb/No and link margin. In addition, atmospheric and rain attenuation, which vary with local climate, both affect the Eb and No of transmitted RE signals.
Furthermore, the system designer must also consider the time varying nature and imprecise knowledge of factors that influence Eb/No and link margin. Thus, it will be appreciated that operators must contend with a wide variety of factors, some of which vary with time, that influence the Eb/No of transmitted RF signals.
Traditionally, approaches that have been used for selecting the appropriate forward length data transmission rate include performing a link budget analysis, performing test measurements at a limited number of points within a geographic region, and test measurements using a mobile test platform. The link budget analysis method is relatively straight forward, but also relatively inaccurate. With this method, link margins are calculated and optimized using published EIRP maps for both the serving (i.e., target) FSS satellite and adjacent satellites, making assumptions about the signals operating on adjacent satellites. The tendency of the analysis is to make conservative assumptions when accurate data is unavailable in order to prevent unintended link dropouts within the desired coverage area. This method does not have a means for compensating for time varying parameters, such as variation in adjacent satellite interference. In some cases, testing by the assignee has revealed that link margins can be several dB higher than those predicted by conservative link budget analysis. This excess link margin represents wasted margin, as it could be converted to increased data rate of the forward link signals.
The second method mentioned above involves taking a test measurement at one or more points within a geographic region. With this method, link margins are calculated at specific locations using test equipment that is representative of the equipment used on actual mobile platforms. These test measurements can be used to calibrate analytical link budget models. By making measurements periodically, it is possible to adjust for time varying factors that can affect the link margin. However, the calibrations are only valid for the point at which they are taken. Point measurements may not capture variations with location. For example, some EIRP patterns from adjacent satellites may overlap a portion of the desired coverage area. If the measurement location is taken outside of the EIRP pattern for a given interfering satellite, the measurement will not register changes in the interference from that particular satellite. In addition, maintaining test rigs at multiple points within a coverage area, and possibly in multiple coverage regions around the world, is extremely expensive.
Finally, the third method mentioned above, that of taking test measurements using a mobile test platform, can resolve many of the technical difficulties of taking measurements at specific points within a geographic region, but is nevertheless a much more expensive method to implement. In this method, a mobile test platform is used to take measurements along selected routes across a desired coverage area. This provides far more points to calibrate an analytical model with, so that it avoids the problems that face the point measurement method described above. However, maintaining and using a mobile platform strictly for testing is also an extremely expensive method for obtaining the information needed for an accurate analytical model. Also, because individual test flights are expensive to perform, it would be difficult, if not cost prohibitive, to use a dedicated test aircraft across designated flight routes with sufficient frequency to capture various time varying effects that could influence the EIRP patterns for such flight routes.
Accordingly, there still exists a need for a system and method for monitoring and adjusting the data transmission rate of a forward link signal received by one or more mobile platforms in a manner which maintains closure of a communication link with each of the mobile platforms, but which still does not result in excessive link margin, and therefore, optimizes the performance of the communication links formed with each of the mobile platforms. There is further a need for such a system and method that can perform adjustments to the data transmission rate of forward link signals on a near real time basis to counter time varying factors that influence the Eb/No and link margin of forward link signals received by the mobile platforms.